The field of this invention relates to conserving energy that is lost through the action of electrical switches (e.g., armature, semiconductor or any other suitable type of switches). More specifically, but without limiting the applicability of the present invention, this invention relates to improving the efficiency of switching voltage regulators. This invention also relates to controlling the voltage across a switch.
In many different electrical circuits, switching action results in various types of currents that would result in energy loss unless the energy in these currents is somehow conserved. One example is a synchronous switching voltage regulator, which includes two switching transistors that are switched ON and OFF out of phase with one another by a control circuit. The switching transistors include a main switching transistor and a synchronous switching transistor. When the synchronous switching transistor is turned OFF in each cycle, the channel current of the synchronous switching transistor moves into its body diode. A short time later, the main switching transistor turns ON, and a reverse recovery current flows through both switching transistors. The reverse recovery current increases rapidly to a large value, causing substantial power dissipation, because the body diode of the synchronous switching transistor has not yet commutated.
One approach to alleviating this problem involves controlling the rate of rise of the reverse recovery current, as disclosed in commonly-assigned U.S. Pat. No. 6,504,351 (hereinafter, xe2x80x9cthe ""351 patentxe2x80x9d) to Eager et al., which is hereby incorporated by reference in its entirety. In the particular embodiments disclosed in the ""351 patent, at least one inductor is placed in the commutation path of the body diode of a switching transistor. The inductor reduces the maximum reverse recovery current through the switching transistor, which reduces power dissipation.
The reverse recovery current flows through the inductor, which means the inductor stores energy. It is highly desirable to transfer this energy back to the input voltage source and/or other places where it may be used. In other words, it is highly desirable to conserve this energy. Possible methods and apparatus for restoring this energy to the input voltage source are described in the ""351 patent and also in commonly-assigned U.S. Pat. No. 6,495,993 (hereinafter, xe2x80x9cthe ""993 patentxe2x80x9d) to Eager, which is hereby incorporated by reference in its entirety. Although the energy transfer methods and apparatus described in the above patents are believed to be highly efficient, at least in some circumstances, it would be desirable to have another type of energy transfer system.
In addition to energy considerations, it is often necessary to control the voltage across a switch to ensure that the switch isn""t damaged. The above patents describe an effective circuit configuration for ensuring that the absolute value of the voltage across a switch does not reach too high a level. In particular, embodiments are disclosed therein that show a loop comprising an inductor, a capacitor and a diode. The switch is coupled to the inductor such that the voltage across the switch is related to the voltage across the inductor. When this voltage reaches a sufficiently large value, the diode turns on, such that current flows through the loop. That is, current flows through the inductor, capacitor and diode, limiting the voltage drop across the inductor to the sum of the voltage drops across the capacitor and diode. Moreover, The voltage drop across the capacitor is controlled, which in effect controls the voltage across the switch.
Although the above patents disclose effective methods and apparatus for controlling the voltage across the capacitor, it would be desirable to provide an alternative to those methods and apparatus, at least in some circumstances.
The switch recovery circuit of the present invention answers the above needs. The switch recovery circuit of the present invention may be used in a circuit that includes a switch, an input energy source and current path circuitry. The current path circuitry may comprise, for example, a transistor, an inductor and an output capacitor that are part of a switching regulator circuit.
The switch recovery circuit of the present invention preferably includes a first inductor, a second inductor, a first capacitor, a first diode, and a recovery circuit. The recovery circuit transfers energy from the first inductor to the first capacitor and to the recovery circuit, and then back to the input energy source and/or to the current path circuitry.
As in the ""351 patent and the ""993 patent, the first capacitor and first diode comprise an AC coupled loop circuit around the first inductor. The first capacitor preferably has a relatively large capacitance value (e.g., 22 xcexcF), so that the voltage across it remains relatively constant over time.
When the switch first opens, the current in the first inductor begins to decrease, causing the voltage across the first inductor to increase. In turn, this voltage increase causes the first diode to conduct and current flows in the loop comprising the first inductor, first capacitor and first diode. The switch is coupled to the input energy source (generally an AC ground) through the first capacitor and the diode so that the voltage across the switch is largely dictated by the voltage across the first capacitor when the diode conducts. Thus, the voltage across the switch may be kept at a relatively low value, which prevents the switch from being damaged.
The above mentioned loop current charges the first capacitor, which represents a transfer of energy from the first inductor to the first capacitor. The energy stored in the capacitor may increase during each switching cycle if the excess charge therein, caused by the loop current, is not returned to the input energy source and/or the current path circuitry. The recovery circuit assists in transferring this energy back into the input energy source and/or to the current path circuitry. The extent to which this energy return occurs during each cycle depends on circuit specific parameters.
The recovery circuit provides current that discharges the first capacitor (i.e. current that flows in the opposite direction to the loop current) after the loop current stops. The loop current stops because, as the current through the first inductor stabilizes, the absolute value of the voltage drop across the first inductor decreases, which in turn shuts off the current through the first diode.
The current flowing from the recovery circuit through the first capacitor may be returned to the input energy source and/or the voltage path circuitry. In either event, energy is transferred from the first capacitor.
The recovery circuit includes a second capacitor coupled at a node to the series combination of a third inductor and a second diode. The second diode is interposed between ground and the third inductor such that current may not flow through the third inductor to ground but only from ground through the third inductor. The third inductor and the first inductor are mutually inductive. Preferably, a voltage across the first inductor corresponds to a multiple of that voltage across the third inductor. In other words, the ratio of windings of the third inductor to the first inductor is preferably N:1, where N is greater than 1.
The action of the third inductor and second diode after the switch is closed depends on the voltage across the first capacitor. As previously mentioned, the voltage across the first capacitor gradually increases each switching cycle if the excess charge caused by the loop current is not returned. When the switch is open and the first diode is on, the voltage across the first inductor is almost equal to the voltage across the first capacitor. Thus, the larger the voltage across the first capacitor, the larger the voltage across the first inductor.
If the voltage across the first capacitor is sufficiently large, the negative voltage across the third inductor, which is equal to N times the negative voltage across the first inductor, will be sufficiently large to lower the voltage at the cathode of the second diode to turn on the second diode, such that current flows from ground through the third inductor. In this case, if the first diode is on, depending on the circumstances, current flows through the third inductor to charge the second capacitor or through the second inductor and the first diode.
If the first diode is off, depending on the circumstances, current flows through the third inductor to charge the second capacitor or through the first capacitor, thereby discharging the first capacitor. As previously described, the discharging of the first capacitor corresponds to a desirable return of energy to the input energy source and/or the current path circuitry.
In addition to returning energy from the first capacitor while the switch is open, the recovery circuit, in conjunction with the second inductor, acts to return energy from the first capacitor when the switch is closed. In this case, the voltage across the first inductor produces a voltage across the second inductor that pulls current across the second capacitor. This current flows through the first capacitor, thereby discharging the first capacitor. Discharging of the first capacitor results in a desirable return of energy from the first capacitor, as previously mentioned.
In addition to providing the energy return function mentioned above, the recovery circuit helps to provide current to the first inductor after the switch has opened but while the first diode is still on. This additional current helps to stabilize the voltage across the first inductor during this time.